Three forms of liver microsomal cytochrome P-450 (P-450) have been highly purified from livers of rats treated with phenobarbital or 3-methylcholanthrene. These P-450s have been partially characterized using a variety of techniques of protein chemistry. P-450 has a subunit molecular weight of about 55,000; 6-9 subunits associate to form a basic aggregate P1. Hydrodynamic measurements indicate that the P1 species further aggregates to form P2, P3....,Pn in a manner well-described by an indefinitely-associating model with k equals (Pn plus 1)/(Pn)(P1), where both k and the molecular weight of P1 are influenced by solvent conditions. Antibodies have been raised to two of the P-450s and used to demonstrate that 1) hepatic and extrahepatic P-450s are immunologically similar, 2) that hepatic microsomal and nuclear P-450s are immunologically similar, 3) that different P-450s are localized in different parts of the liver and 4) the human and rat P-450s share common antigenic determinants. Multiple forms of epoxide hydratase appear to be present in both rat and human liver; these various forms can be distinguished by amino acid composition, electrophoretic mobility, and immunochemical reactivity. The various forms differ somewhat in rates of hydrolysis of carcinogenic epoxides and are preferentially induced and repressed by various xenobiotics. Alkyl pyrroles and vinyl chloride are activated to alkylating agents by P-450s; in the latter case, an epoxide appears to be formed.